Thermochemistry of organic and heteroorganic species. Pad VIII. Ketene andstructurally related species

Photoionization mass spectrometry was used to obtain the enthalpies of form ation of CH2=C=O less than or equal to -20.5 and -24.0 kcal mol(-1) (from 3 -phenylcyclobutanone and diketene, respectively), CH3C equivalent to O+ ion 147.3 and 152.3 kcal mol(-1) (from CH3COOCH=CH2 and CH3CONH2 molecules, re spectively) and PhCH2C equivalent to O+ 175.5 kcal mol(-1) (from PhCH2COOMe molecules). The enthalpic shift procedure was applied for the estimation o f the enthalpies of formation of ketene and related molecules. The followin g Delta H-f(0) values were found CH2=C=O: -(22-25), CH2=S=CH2, (67), CH2=S= S (60), S=S=S (51), HC-COH (10 kcal mol(-1)). The low value of Delta H-f(0) (ketene) approximate to -23 kcal mol(-1) as compared with the currently us ed value -11.4 kcal mol(-1) was supported by the literature data, which hav e been revised in the present work. Using the new value for ketene's enthal py of formation, those for ten substituted ketenes and HC equivalent to CO- (48.5 kcal mol(-1)) free radical were obtained with the help of macroincre mental schemes and introduction of correction terms. Computation of the ent halpies of formation of eight A=B=C molecules by MNDO, AMI, PM3 and MINDO/3 methods revealed that in most cases the latter gives the results closest t o the experimental values or to those gained from the enthalpic shift proce dure. The brief analysis of possible sources of errors in deducing the ther mochemical values from appearance energies measurements has been made. Amon g those the isomerization processes occurring in molecular ions are conside red the most important, which could lead to incorrect values of the enthalp ies of formation of fragment ions, free radicals and molecules. With many e xamples it has been demonstrated that the application of the series of isod esmic reactions could be an effective tool for verifying, correcting and fi nding new values of the enthalpies of formation of neutral and ionized spec ies. (C) 2000 Elsevier Science B.V. All rights reserved.